Dispersion of a material is its variation of refractive index (n) with respect to wavelength (λ). For visible light, most transparent materials have the following relationship between the changes in its index of refraction (n) verses wavelength (λ):
            ⅆ      n              ⅆ      λ        <  0.
It follows from this relationship that as the wavelength (λ) increases, the material's index of refraction (n) decreases. Thus, at a material interface, such as air-to-glass, the angle at which light is refracted will vary with wavelength, causing an angular dispersion of colors. For imaging over a wide spectrum, such as the visible, this introduces chromatic dispersion. This shows up in imaging as chromatic aberration, which degrades performance and image quality of the lens. Thus, lowering the dispersion in an optical design can greatly improve the overall image quality that a lens provides.
One way to define a material's dispersion properties in the visible region is known as the Abbe Number (or V-Number). The Abbe Number of a transparent material can be represented in a number of ways, the most common in many applications defines the relationship of the indices of refraction (n) in a given material as:
      V    d    =                    n        d            -      1                      n        F            -              n        C            where d is the yellow Fraunhofer helium-d line (at a wavelength of 587.6 nm), while F and C are the Fraunhofer spectral lines (486.1 nm and 656.3 nm respectively). This relationship shows that materials with a high Abbe Number (Vd) have low dispersion, making them attractive in optical designs that require chromatic dispersion correction.
Further, an Abbe Diagram or “glass map” can be constructed by plotting Abbe Numbers (Vd) of materials versus refractive index (nd). Such diagrams or maps are typically provided by manufacturers in the field. This allows materials, and more commonly glasses, to be easily categorized based on their properties and composition. For example, flint glasses are defined as those having Vd<50, with a very dense flint glass having Vd around 20. Crown glasses are defined as those having Vd>50, with very light crown glasses having Vd values up to 65. In order to achieve even higher Abbe Numbers and, thus, lower dispersion in glass form, it was necessary to introduce fluoride compounds into glass melts to allow glass to approach the optical properties of fluorites. Fluorites, such as Calcium Fluoride (CaF2), Lithium Fluoride (LiF), etc., typically have very high Abbe Numbers, but are crystalline in structure, which tends to make them very soft and susceptible to fractures and etching. So, while their optical properties are desirable, mechanically they are not desirable when lens surfaces are exposed to the outside environment. This resulted in glass manufacturers developing fluorite-like glass, creating a new category of glass called fluor-crowns or Extra-low Dispersion (ED) glass. They have the same favorable optical properties of fluorites in that they have low dispersion, but do not have a crystalline structure. So, they exhibit improved mechanical properties over fluorites and are less susceptible to fractures and harsh environments. These Extra-low Dispersion glasses have come to be defined as glasses with an Abbe Number Vd>80, the same region that most fluorites fall under. The Japanese glass manufacturer Ohara Corp., as an example, currently produces three types of Extra-low Dispersion glass: S-FPL51 (nd=1.497, Vd=81.6), S-FPL-52 (nd=1.456, Vd=90.3), and S-FPL-53 (nd=1.43875, Vd=95.0).
In the field of Ophthalmology, it is well known that to perform a standard examination of a patient's internal eye structure, particularly the retina or fundus, the examiner typically uses a diagnostic ophthalmic lens in conjunction with either an indirect ophthalmoscope or a slit lamp biomicroscope.
U.S. Pat. No. 4,627,694, “Indirect Ophthalmoscopy Lens for Use With Slit Lamp Biomicroscope” describes a diagnostic ophthalmic lens design comprising a singlet made of a homogeneous transparent optical material having two aspheric surfaces of revolution, for use in conjunction with a Slit Lamp Biomicroscope. This patent is incorporated herein by way of reference.
U.S. Pat. No. 4,738,521, “Lens for Indirect Ophthalmoscopy” also describes a diagnostic ophthalmic lens design comprising a singlet made of a homogeneous transparent optical material having two aspheric surfaces of revolution, for use in conjunction with an Indirect Ophthalmoscope (either monocular or binocular). This patent is incorporated herein by way of reference.
These patents describe a basic concept for diagnostic ophthalmic lens designs: single lens elements made of a transparent optical material, for use in conjunction with indirect ophthalmoscopes and slit lamp biomicroscopes, in order to obtain an image of a retina or fundus for the purpose of performing an examination on a patient's eye. The diagnostic ophthalmic lenses cited must be placed (hand held or otherwise temporarily mounted) a defined and suitable working distance from the cornea of the patient's eye in order to perform two primary functions. First, it operates as a condensing lens converging light from a source found on either the indirect ophthalmoscope or the slit lamp biomicroscope, into the patient's eye, through the pupil, thereby illuminating the patient's retina. Secondly, the diagnostic ophthalmic lens forms an indirect image, located in a plane external to the eye structure itself, of the patient's retinal surface. This indirect image of the curved retinal surface forms a generally flat image plane, typically free of any significant image aberrations, wherein it becomes available for convenient observation by the examiner using either an indirect ophthalmoscope or a slit lamp biomicroscope.
The performance of these lenses, however, could be improved. In this regard, the presently described embodiments relate to an improved diagnostic ophthalmic lens and/or lens assembly or device.